Build only required class in Android Junit Testing

I am working on a project which takes around 2-3 minutes to build. So to write basic util functionalities i thought it might be better to use android unit testing for debugging and testing.

So I read about it and trying to implement it. But now these unit cases are also taking the same time around 1-2 min. When I run a test case it compiles the whole project that's why it take so much time.

Is there any other way to compile only required classes or any thing that I am doing wrong?

P.S: there are several other reasons why we are implementing unit test case not only this.